1. Field of the Invention
The present invention relates to a power amplifier, specifically to a push-pull high output, high frequency power amplifier operating at a high efficiency in a microwave band.
2. Description of the Related Art
A conventional high frequency power amplifier will be described.
Conventionally, a push-pull power amplifier circuit for operating two power amplifying elements (for example, FETs) in reverse phases from each other and synthesizing output signals from the power amplifying elements to provide an output signal is known.
FIG. 4 is a block diagram of a conventional high frequency power amplifier 400 disclosed in Japanese Laid-Open Publication No. 5-29851.
In FIG. 4, a signal input from an input terminal 401 is divided into two signals by a power divider 402. The two signals have reverse phases from each other and have the same amplitude as each other. The two signals are input to a pair of FETs 405 through input-side matching circuits 403, respectively. The pair of FETs 405 are connected in parallel. A gate of each FET 405 is biased through a resistor 404, and a drain of each FET 405 is biased through a coil 409. A distributed constant line 406, provided in connection with an output of each FET 405, is connected to a power synthesizer 411 through a capacitor 410. The distributed constant lines 406 are connected to each other, through a capacitor 407 for controlling third-order harmonic components, at a point of each distributed constant line 406 which is a distance away from the output of the corresponding FET 405, the distance being 1/12 of the fundamental wavelength to be amplified. The distributed constant lines 406 are also connected to each other, through a capacitor 408 for controlling fundamental wave components, between the connection points of the distributed constant lines 406 and the capacitor 407, and the power synthesizer 411.
The two signals having the reverse phases and the same amplitude and amplified by the pair of FETs 405 are input to the power synthesizer 411 through an output-side matching circuit 420. The matching circuit 420 includes the distributed constant lines 406, the capacitor 407, the capacitor 408, and capacitors 410. Then, the signals are synthesized and output to an output terminal 412.
In such a conventional high frequency power amplifier 400, a load impedance with respect to the third-order harmonic components at the output of the FET 405 becomes as high as open, e.g., nearly 100 .OMEGA., due to the distributed constant lines 406 and the capacitor 407. Thus, the pair of FETs 405 operate at a high efficiency.
In this specification, the expression "as high as open" refers to "as high as, for example, nearly 100 .OMEGA.".
A high frequency power amplifier for transmitting signals used in wireless communication devices such as, for example, cellular phones is generally demanded to have a high output and a high efficiency in operation. In order to make maximum use of the inherent characteristics of the power amplifying elements used in a high frequency power amplifier, the load impedance needs to have a value in a limited range at an input and an output of the power amplifying element. Accordingly, very fine adjustments are required to configure a matching circuit.
In the conventional high frequency power amplifier 400, the capacitor 408 for controlling the fundamental wave components and the capacitor 407 for controlling the third-order harmonic components are connected to the same distributed constant lines 406. In such a structure, control of the load impedance at the outputs of the power amplifying elements is complicated, thereby making it difficult to optimize the load impedances with respect to both the fundamental wave components and the third-order harmonic components.